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91 s10 blazer 4x4 front axle

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oakman

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Discussion starter · #1 ·
hello all.
i have a 91 blazer 4x4, 4.3, with 300,000 miles and, miraculously, still going strong on all original drivetrain and motor. several years ago we recieved a notice from chevy regarding the antilock system, i think it was, can't really remember now, saying take it to a dealer for free repair. anyways, about the same time, the front axle would occasionally try and engage while going down the road, with a big clunk. we took it in and they fixed it by adding some type of vacuum switch thing which they mounted up on the right fender in the engine compartment. i've worked on trucks for more than 30 years and do all my own repairs and maintenance. i was just wondering exactly what that thing is and what it does, if any one knows, i'd appreciate it. thanx
 
#3 · (Edited)
Where did you have this done? 91's had a recall for one of the 4x4 wires, free dealer repair. I had it done, even though I had no problems.

I had some anti-lock brake problems, and never heard about a recall though, whats the deal with that?

I ended up just unplugging them.
 
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#5 ·
oakman said:
hello all.
i have a 91 blazer 4x4, 4.3, with 300,000 miles and, miraculously, still going strong on all original drivetrain and motor. several years ago we recieved a notice from chevy regarding the antilock system, i think it was, can't really remember now, saying take it to a dealer for free repair. anyways, about the same time, the front axle would occasionally try and engage while going down the road, with a big clunk. we took it in and they fixed it by adding some type of vacuum switch thing which they mounted up on the right fender in the engine compartment. i've worked on trucks for more than 30 years and do all my own repairs and maintenance. i was just wondering exactly what that thing is and what it does, if any one knows, i'd appreciate it. thanx
Click to expand...
On the front passenger fender, between the fender and battery/coolant tank is an electrical switch thrown by a set of switcheswhen you engage the xfer case that causes vacuum to suck on the actuator on the transaxle.

btw - when the climate control head goes bad, vacuum leaks on the positition switch and under throttle the S10 will drop 4x4.
 
Discussion starter · #6 ·
quaddriver,
the front differential is engaged by a vacuum servo, located under the battery tray, connected to the diff by a cable. the way this happens is by a 3 port vacuum switch located on top of the transfer case. this is all done mechanically, i.e. you engage the transfer case, the 3 port applies vacuum to the servo, and engages the front diff. the only electrics involved are for the indicator lights on the transfer case shift console, and antilocks. the item you are reffering to was added by the dealer, and as stated previously, is a vacuum switch. it's function is to tell the antilock control module whether it is in 2, or 4 wheel drive. i previously stated that i thought maybe this had something to do with my trucks front diff trying to engage on it's own going down the road, but i realize now that it did not. the only possible explanation is the 3 port switch was faulty, i.e. leaking vacuum to the servo. i hear you on the leaking climate control selector, however mine has leaked for years, only on defrost and a/c and does not cause a loss of 4x4. a more likely reason would be deteriorated vacuum lines. the most common occurence of no 4x4 in these trucks is a faulty/rotted front diff vacuum servo. i have also had problems with transmission fluid from the transfer case leaking past the 3 port switch and accumulating in the vacuum lines. however a quick blow out with air fixes it. thanx
 
#7 ·
oakman said:
quaddriver,
the front differential is engaged by a vacuum servo, located under the battery tray, connected to the diff by a cable. the way this happens is by a 3 port vacuum switch located on top of the transfer case. this is all done mechanically, i.e. you engage the transfer case, the 3 port applies vacuum to the servo, and engages the front diff. the only electrics involved are for the indicator lights on the transfer case shift console, and antilocks. the item you are reffering to was added by the dealer, and as stated previously, is a vacuum switch. it's function is to tell the antilock control module whether it is in 2, or 4 wheel drive. i previously stated that i thought maybe this had something to do with my trucks front diff trying to engage on it's own going down the road, but i realize now that it did not. the only possible explanation is the 3 port switch was faulty, i.e. leaking vacuum to the servo. i hear you on the leaking climate control selector, however mine has leaked for years, only on defrost and a/c and does not cause a loss of 4x4. a more likely reason would be deteriorated vacuum lines. the most common occurence of no 4x4 in these trucks is a faulty/rotted front diff vacuum servo. i have also had problems with transmission fluid from the transfer case leaking past the 3 port switch and accumulating in the vacuum lines. however a quick blow out with air fixes it. thanx
Click to expand...
you described pretty much what I did, cept the only vacuum on my 91 is at the servo on the fender, the switch on my xfer case is electrical.

lemme guess, you have a 2 door? it seems you are describing the interface of kelsey hayes RWAL system, I have a KH 4WAL system - its does not care if 4x4 or not, but the RWAL will be disabled if in 4x4....it was my understanding 4WAL was std on 4 doors and optional on 2 doors
 
#9 ·
oakman said:
no it's a 4 door with 4wal. so yours doesn't have the 3 port vacuum switch up on top of the t-case? how does vacuum get applied to the servo when you engage the t-case?
Click to expand...
well ok, u pull the lever, and the pin switch on the case throws the DIC light from 2wd to 4wd and engergizes the vacuum solenoid on the fender. Vacuum ported from the same T where the HVAC gets its vacuum then begins sucking on the transaxle.

When the collet slides, a second pin switch in the transaxle then throws on the 'body diagram' light on the DIC and away I go. Its real similar to how the all electric system on the bigger trucks work.
 
Discussion starter · #10 ·
so you are saying that instead of the mechanically operated 3 port vacuum switch, which is located on the very top of my t-case(np203), yours has an electrically operated vacuum switch? where is it, and what t-case do you have?
 
#11 ·
oakman said:
so you are saying that instead of the mechanically operated 3 port vacuum switch, which is located on the very top of my t-case(np203), yours has an electrically operated vacuum switch? where is it, and what t-case do you have?
Click to expand...
Actually you may be right, I had a copy of a K truck chassis wiring diagram in my s/t folder....now wonder it looked the same. But I wont be at the shop till friday to crawl underneath and check mine out...
 
Discussion starter · #12 ·
i've worked on several 91's, and wasn't aware of any different system than whats on mine. i'd bet yours is the same. there are no electrically operated vacuum switches that i'm aware of on these trucks. the 3 port is a mechanicaly actuated vacuum switch. there are 3 lines into it. one from the vacuum reservoir. one to the vacuum servo for the front diff, and one bleed line, to relieve the vacuum when you shift out of 4x4. the device on the passenger fender is not a factory installed item. there was a recall for it. i knew what it was when the dealer did it, but had forgotten, thus my initial post and question regarding it's function. all it does is close when vacuum from the 3 port is applied, and tells the abs module that it's in 4x4. at least thats how i understand it. the curious thing is that the truck's front diff was trying to engage on it's own while driving. with a big clunk, obviously, cause the t-case wasn't engaged. the only thing i can think of is that the 3 port was leaking vacuum to the servo. as far as i know they didn't replace it, but after they added the switch on the fender well, it quit doing it, so they may have installed a new 3 port. oh well, thanx for your input. like i said, i can't believe this little truck has 300,000 miles on the original 4.3 motor and drivetrain and shows no sign of quitting , yet. i drove a 72 chevy van for 16 years and 450,000 miles, but that was on 2 motors, and about 6 trannys. wonder if our 98 blazer will go that far. thanx again.
 
#13 ·
Where is teh climate control switch located? I have the Helm manuals for the 1997 S/T trucks and can't find it in there. Also I can't find the 3 port vacuum switch in the book either, where exactly is that switch located? My TCCM is functioning properly but I still don't have 4WD so what you aretalking about on this page sounds like teh problem I have.

Thanks
 
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